The stock passive cooler of Rock5B is enough for light to medium loads, but as for this KKSB case, passive cooling is not sufficient under heavy loads.
I only tested the stock Radxa 40x40x25 blue aluminium cooler, but with a 40mm 5V fan on top, and this already made a significant difference, with temperatures below 70°C at full load. Also, if thermal paste can be used (I did not test), we could probably gain a few degrees.
Recently, I switched to Rock5A with a custom active but quiet cooling solution for light/medium loads in everyday use (as Minecraft server and as general access Armbian workstation) inside the aluminium Radxa EcoPi Pro case.
Finally, I plan to use my Rock5B as NAS and video surveillance station, where the passive cooler is placed in the air flow of the main 120mm extraction fan of a Fractal Design 304 NODE mini ITX case. This way, I only rely on a single fan with long life expectancy, as the solution will have to run continuously over several years… and I hope it will be able to maintain temperatures below critical levels until its retirement